Rebalancing electromagnetic servo system



y 1952 w. w. SHANNON REBALANCING ELECTROMAGNETIC SERVO SYSTEM FiledSept. e, 1946 w ill... R mm an. .NNAWI In w M .0 my a! lo 0 m 5.54mi fizEr=dz m P9 IO 0 Wk g 3 on non E m 8.

Patented July 8, 1952 REBALANCING ELECTROMAGNETIC SERVO SYSTEM WilliamW. Shannon, Glen Cove, N. Y., assignor to Fairchild'Camera andInstrument Corporation, a corporation of Delaware Application Septembere, 1946, Serial No. 695,329

This invention relates'to rebalancing electromagnetic servo systems and.while it is of general application, it is particularly suitable forembodiment in a rebalan'cing' automatically indicating scale and will bespecifically described in such an application.

Heretofo're there: have been proposed" various types of electricallyoperated automatic indicating 'fs'cales. However, the majority of thesescales have been of the deflection type, that is, a type in which theapplication of a weight unbalances a normally balanced system and'theamount'of unbalance is indicated as a measure of the applied weight. Theprincipal disadvantage of this type of scaleis the difficulty ofdesigning a system in which the response is linear withres'pect toweight overfthe complete deflection range. It is also difficult toobtain a wide'defiection range and substantially uniform sensitivityover the range. v

v There have also been proposed certain so-called automatic rebalancingelectromagnetic scales, but these have not metwith commercialsuccess dueto a number of disadvantages and limitations. For' example, while theyhave generally been of the rebalancing type, they have also been of theposition type. that is, a type in which a deviation of the system fromthe balance point initiates a correcting action which tends to reducesuch deviation but never eliminates itcompletely, since such eliminationwould remove the correcting action necessary to maintain the balanceunder the new "condition; In otherwords, systems of this type have beensubject to a substantial and undesirable regulation characteristic It'isan object of the present invention, therefore, to provide a new andimproved rebalancing electromagnetic servo system which avoids one ormore of the above-mentioned disadvantages and limitations of. the priorart systems.

It is another object of the invention topr ov'ide 12 Claims. (c1.265-40) a new and improved automatically indicatingrebalancing scalewhich obviates one or more of the above-mentioned disadvantages andlimitations of the prior art automatically indicating scales balancingscale in which there is developed automatically an indication accuratelyand linearly representative of the applied weight.

*afurther objector the invention to provide a new and improvedautomatically indicating re- It isa still further object of theinventionjto provide a new and improved automatically indicatingrebalancing scale in which the scaleis always accurately rebalanced toits normal or neutral position and in which the magnitude of thecorrection effect necessary to rebalance the scale is accurately andlinearly representative of the applied weight.

mechanically 'coupled'to a movable member of the actuating device and anintegrating device coupling the control device to such exciting windingto maintain the movable member in a predetermined equilibrium position.The system further includes means for modifying the signal input to theintegrating device from the control device, the energization of theexciting winding being representative of the effect of the modifyingmeans. When the system is embodied in a rebalancing scale, the means formodifying the signal input to'the' integrating device comprisesweight-supporting means connected to the movable member and theactuating device. By the term rebalancing as applied, to a servo systemor scale is meant an apparatus which is, at equilibrium, always returnedto the same zero or null position.

For a better understanding of the invention, together with other andfurther object thereof, reference is had to the following descriptiontaken in connection with the accompanying drawing, while its scope willbe pointed out in the appended claims.

Referring now to the drawing, the singlefigure represents, partiallyschematically, a complete high permeability material having a lowremanence and a low hysteretic constant, such as soft steel. The systemalso includes a signal-control device ll having movable elementmechanically coupled to the movable member of the actuating device.Specifically, the movable portion of the device ll comprises a beam Ilapivotedat I lb and connected at one end to the movable armature [0c andhaving at its opposite end an electromagnetic pick-up device responsiveto the position of the beam. This pick-up device comprises asector-shaped armature llc forming'the opposite end of the beam I la andmeans for developing a periodic output: signal varying in phase withvariations in the sense of the deviation of the movable beam i In from apredetermined normal or balanced position. The signal-developing meanscomprises a normally balanced E-magnet IId having an exciting windinglie mounted on its center leg and connected to suitablealternating-current supply terminals I2. The magnet II (1 also includesa pair of pick-up windings II and Ilg disposed on the outer legs of theE-magnet and connected in series opposition.

The servo system further includes an integrating device coupling thecontrol device II to the exciting winding ID!) of the actuating deviceIt] to maintain the movable scale beam Na in a predetermined equilibriumor balanced position. This integratmg device includes a sensing circuitI3 for developing a unidirectional signal varying in polarity with thephase of the output signal of the control device II. The circuit I3 isprovided with input terminals 13a, I3 a, connected to the pick-upwindings of the control device II through an amplifier 9 which may be ofconventional design and also energized from the supply circuit terminalsI2. The input terminals I3a, I3a are connected to the primary winding ofa transformer I4 having secondary windings Ida and Mb for developingsignals of opposite polarity. The sensing circuit I3 comprisesessentially a bridge circuit, one branch of which includes the winding Iia, a diode rectifier I5a, and a secondary winding I6a of a transformerIS, the primary winding of which is energized from the supply circuitterminals I2, and a load circuit comprising a resistor I la and a shuntfilter condenser I8a. The other branch of the bridge circuit similarlycomprises winding MIL-a diode rectifier I5b, secondary winding I62) oftransformer I6, and load resistor I'lb shunted by filter condenser I811.The algebraic sum of the unidirectional signal potentials appearingacross resistors Ila. and I'lb in series is applied to the outputterminals I 3b, I3b of the sensing circuit I3.

The integrating device of the servo system also includes an integratingcondenser I9 and a pair of normally nonconductive devices, such as thetriode vacuum-tube impedances and 2|, connected individually to chargeand discharge, respectively, the condenser I9 from a suitable source,such as the :3 supply circuit terminals 22. The tube 20 is provided withan anode load resistor 23, while the tube v2i is provided with a cathodeload resistor 24. The tubes 20 and 2| and their respective loadresistors 23 and 24 are preferably so proportioned that, when connectedacross the supply terminals 22 and with no signalsapplied to theircontrol grids the terminal 25 common to the cathodeof tube 20 and theanode of tube 2| is normally approximately at unidirectional groundpotential and the circuit described is symmetrical with respect toground.

7 Suitable signal by-pass condensers 26 and 21 may be connected betweenthe :3 terminals and ground.

The automatically indicating scale further includes means for energizingthe exciting winding Iflb of the actuating device II] in accordance withthe charge on the condenser I9. This means may comprise acathode-follower repeater comprising a triode vacuum tube 28 having itscontrol electrode connected to the ungrounded terminal of condenser I 9and provided with a catho'de load resistor 29, the anode of the tube '28being energized from a suitable source +3. The signal .appearing acrossthe resistor 29 is applied toinput-terminals 30a of a power amplifierrepresentative effect.

30 having output terminals 3% connected to the exciting winding lb ofactuating device ID. The power amplifier 3!] is also energized fromsupply circuit terminals I2.

ably connected to and supported by the movable armature I00. Theplatform 3! is adapted to support any object 32 to be weighed. With thisarrangement, as explained in more detail hereina'fter, the energizationof the exciting winding I0]; is representative of the effect of theabovedescribed modifying means, that is, of the weight of the object 32.Therefore, the system also includes means responsive to the energizationof the exciting winding Ifib for developing such "a This responsivemeans may be in the form of a current indicating means or ammeter 33connected in series with the exciting winding IIlb and having a movablepointer 33a and associated scale 33gb calibrated to develop anindication of the magnitude of the modifying action, that is, of theforce applied to, or the weight of, the object 32, on the supportingplatform 3|. It is believed that the operation of the abovedescribedautomatically indicating rebalancing scale will be clear from theforegoing description. In brief, it will beassumed that the system isinitially balanced in the absence of the object 32 to be Weighed andthat suificient cur- I compensate for any unbalance in the movable scalebeam Ho. and its connected elements to maintain the beam in its neutralor balanced position. Under these conditions, the armature No of thesignal-control device effects an equal coupling between the excitingwinding II 6 and the two pick-up windings II and Hg. Due to theirconnection in series opposition, the resultant signal supplied by thesewindings to the amplifier 9 is zero. Therefore, no signal is applied bythe transformer I 4 to the twoarms of the bridge circuit, while equalsignals are supplied by the transformer IE to the two branches of thebridge circuit, and equal and opposite unidirectionalsignals appearacross the load circuits Na, [8111 and 17b, I8b under these conditions.The circuit constants are so proportioned that, with these signals, thegrids of the vacuum tubes 28 and" 2| are biasedslightly beyond cutoff sothat these tubes are nonconductive and constitute essentially infiniteimpedance devices. Therefore,

the terminal 25, connected to the ungrounded terminal of condenser I9 issubstantially electricallyisolated and any charge existing on thecondenser I9 is maintained with inappreciable leakage. Therefore, thesignal developed across the load resistor 29 of repeater tube 28 is alsomaintained constant and similarly the excitation of the winding IIJb,thereby to maintain the system in equilibrium. For these conditions thescale 33b is calibrated to read zero.

If now a force be applied to the platform 3!, as by the disposition ofan object 32 thereon, the scale beam Ila becomes unbalanced, deflectingin a clockwise direction. The armature He thus increases thecouplingbetween the exciting. winding He and the pick-up winding II anddecreases that between the windings He and I lg, thus applying to theamplifier 9a signal-of a predetermined polarity. This signal isamplified conductive, the integrating condenser f charged therethroughfrom the source +B so that the terminal 25 becomes more positive.

Thecharging of the condenser l9 continues as 'longxas there is anyunbalance of the scale beam .I la and the final charge of condenser I9is proportional to the time-integral of the signal outthe weight of theobject "32.

in amplifier 9' and applied to the'sensing circuit 13.1 The transformerH'applies-thissignal with one polarity to'the branch of the bridgecircuit including the rectifier l5a and with opposite "polarity to thebranch of the circuit including rectifierWEb. The phase connections aresuch thatthis "signal adds tothat-supplied by the transformer l6 intheupper branch of the bridge circuit and subtracts from that suppliedbythe 1' transformer [6 in the lower br'anch of the bridge circuit.Thusfthe unidirectional potentialappearing across the lo ad circuit [1a,[8a increases "positively, while that appearingacross the load circuitI11), I81) decreases positively. Therefore,

the repeater tube 20 is rendered conductive but there is no effect uponthe tube 2|, the grid of which is made more negative, since this tubeisalready nonconductive. Asthe tube'20 becomes i 9- is put from the deviceI I applied to the amplifier 9. Therefore, the grid of the repeater tube28 is also made more positive to increase its conductivity and toincrease the signal developed across the resistor 29 and applied to thepower amplifier 30. The signal output of the amplifier 30 iscorrespondingly increased to increase the energization of the excitingwinding b of the actuating device [0 and this operation continuesuntilthe energization of this winding is sufiicient to overcome theeffect of the weight 32 and to attract the armature I00 back to itsnormal or balanced position. When this condition occurs, the controlsystem described becomes inoperative, as explained above, and the newvalue ofcharge on the condenser 19 and the'new'value ofenergization ofexciting winding [0b are maintained.

The current indicator' 33 in series withthe winding lUb indicates on itscalibrated scale'33b Obviously, if the object 32 is removed or anotherobject oflesser weight is substituted the reverse of the operation justdescribed takes place, reducing the energization of the winding 10b andthe indication of the meter 33.

Thus it is seen that, in the automatically indicating rebalancing scaleof the invention including the integrating feature, the. system is re-,balanced only when the scale beam I la is in exiactly the neutraljorbalanced position and that this balance is maintained until disturbed bythe change of the weight of the object 32, no

control signal from the device ll being required to maintain the systemin its new condition of balance. Thus, the system has a zero regulationcharacteristic and its accuracy is determined solely upon thesensitivity of the various electrical responsive and actuating devices.

While there has been described what is at present considered to be thepreferred embodiment of the invention, it will be apparent tothoseskilled in the art that various changes and: modiflcations may be madethereinwithout depart- ;ing, from the invention, and it is, therefore,aimed in the-appended claims to cover all such. changesand-modifications as'fall within the true spirit and scope of theinvention.

What is claimed is:

1. A rebalancing electromagnetic servo system comprising, anelectromagnetic actuating device including relatively movable field andarmature members at least one having an exciting winding, asignal-control device having a movable element mechanically coupled to amovable member of said actuating device, an integrating device couplingthe signal output of said control device to said exciting winding tomaintain said movable member in a predetermined equilibrium position,and means acting upon said control device for changing its signal outputapplied to said integrating device, the energization' of said excitingwinding being representative of the effectof said last-named means.

2. A rebalancing electromagnetic servo system comprising, anelectromagnetic actuating device including a stationary field memberhaving .an

exciting winding and a movable armature, a signal-control device havinga movable element mechanically coupled to said movable member of saidactuating device, an integrating. device coupling said control device tosaid exciting'winding to maintain said movable member in a predeterminedequilibrium position, and means acting upon said control device forchanging its signal output applied to said integrating device, theenergization of said exciting windingbeing representative of the effectof said last-named means; 1 I p 3. A rebalancing electromagnetic servosystem comprising, an electromagnetic actuating device including"relatively movable field and armature members at least one having anexciting winding, a signal-control device including a pivoted beamconnected at one end to a movable member .of said'actuating device andan electromagnetic pick-up'device responsive to the position of saidbeam, an integrating device coupling said control device to saidexciting winding to maintain said movable member in a predeterminedequilibrium position, and means acting upon said control device forchanging its signal output applied to said integrating device, the

energization of said exciting winding being representative of the effectof said last-named means. I

4. A rebalancing electromagnetic se'rvo system comprising, anelectromagnetic actuating device including relatively movable field andarmat'ure members at least one having an exciting winding, asignal-control device including a pivoted beam connected at one end to amovable :member of said actuating device and an armature memberconnected to the other end thereof and a normally balancedelectromagnetic pick-up device having pick-up winding means associatedtherewith, an integrating device coupling said control device to saidexciting winding to maintain'said movable member in a predeterminedequilibrium position, and means acting upon said control device forchanging its signaloutput applied to said integrating device, theenergization of said exciting winding being representative of the effectof said last-named means.

5. A'rebalancing electromagnetic" servo system comprising, anelectromagneticactuating device including relatively movable field andarmature members at least one having an exciting winding, asignal-control devicein'cluding a pivoted beam connected at one end to amovable member of said actuating? device and an armature memberconnected to the other end thereof and an E-magnet pickup comprising anexciting winding and a pair of pick-up windings connected in seriesopposition, an integrating device coupling said control device to saidexciting 2&925

Winding of said actuating device to maintain said movablemember inapredetermined equilibrium position, and means acting upon said controlexciting winding of said actuating device being ..representative of theeffect of said last-named means. V

6. A rebalancing electromagnetic servo system .comprising, anelectromagnetic actuating device including relatively movable field andarmature members at least one having an exciting'winding, asignal-control device having a movable element mechanically coupled to amovable member of said actuating device and means for developing aperiodic output signal varying in :phase with variations in the sense ofthe devia- ..tion of said movable member from a-predetermined normalposition, an integrating device responsive to said output signal andincluding an integrating condenser, a pair of normally nonconductivedevices connected individually to charge and discharge said condenser,and a circuit for selectively rendering said devices conductive inaccordance with the phase of the output signal of said control device,and means for energizing said exciting winding in accordance with thecharge on said condenser to maintain said movable member in apredetermined equilibrium position, and means acting upon said conminednormal position, a sensing circuit-for deriving a unidirectional controlsignal varying in polarity with the phase of said output signal, an

integrating device coupling said sensing circuit .:to said excitingwinding to maintain said mo v able member in a predeterminedequilibriumpoi sition, and means acting upon said. control device forchanging its signal output applied to'said integrating device, theenergization of .said-ex citing winding being representative of theeffect of said last-named means.

8. A rebalancing electromagneticservo system comprising, anelectromagnetic actuating device including relatively movablefieldandarmature members atleast one having anexciting winding, asignal-control device having a movable element mechanically coupled to.a movable member of said actuating device,- an integrating'devicecoupling said control device to saidexcitingwinding to'maintain saidmovablemembe'rinba predetermined equilibrium position, means acting uponsaid control device for changing itssign'al output applied to saidintegrating device, ;and means responsive'to the energizationo'f saidexciting winding for developing anreifect representative of the actionofzsaidlast-n'a'med means.

9. A rebalancing electromagneticservo system comprising anelectromagnetic actuating device including relatively movable field andarmature members at least one having an exciting winding, asignal-control device having a movable'element mechanically coupled to amovable member of saidactuating device, an integrating devicecou- Ipling said control device to said exciting winding to maintain saidmovable member in a predetermined equilibrium position, means actingupon said control device for changing its signal output applied to saidintegrating device, and current-indicator means connected in series withsaid exciting winding for developing an indication of the magnitudeofthe action of said lastnamed means.

10. An electromagnetic rebalancing forcemeasuring apparatus comprising,an electromagnetic actuating device including relatively movable fieldand armature members at least one having an exciting winding, asignal-:control device having a movable element mechanically coupled toamovable member of said actuating device, an integrating device couplingsaid control device to said exciting winding to maintain said; movablemember in a predetermined -.equilibrium position, and force-applyingmeans connected'to said movable member of said'actuating device, theenergization of said excitingwinding being representative of the forceon said force-applying means.

11. An electromagnetic rebalancing scale comprising, an electromagneticactuatingdeviceiineluding relatively movable field and'armature membersat least one having an exciting winding, a signal-control device havinga movable element mechanically coupled to a movable member ofsaidactuating device, an integratin device coupling said control device tosaid exciting winding to maintain said movable member in a predeterminedequilibrium position, and weight-supporing means connected to saidmovable member of said actuating device,.the energization of saidexciting windingbeing representative of the weight-on said supportingmeans.

12. An electromagnetic rebalan'cing. scale comprising,an-electromagnetic actuating device.ineluding relatively movable fieldand armature members at least one having an exciting winding, asignal-control device having, a movable element mechanically coupledto'amovable member of said actuating device, an integrating devicecoupling said control device to'said exciting winding .to maintain saidmovable memberin a predetermined equilibrium position, weightsupportingmeans connected to .said 'movable member of saidactuating device, andcurrentindicator means connected in series with said exciting windingfor developing an indication of the weight on said supporting means.

' WILLIAM W. SHANNON.

REFERENCES CITED The following referencesare of recordinthe file of thispatent:

Fisher et a1 Mar. 6, 1945

